This happened when I was 14 at the lake where my family vacationed every year.
My great-grandmother owned a large lakeside property with a number of houses and cabins. This meant that many members of our extended family could be accommodated at any one time. It was not uncommon for friends or distant relatives to drop by.
One hot August day some friends of one of my relatives came by. The parents stayed for a few hours and then left with their son, but arrangements were made for their 14-year-old daughter to stay overnight with one of my cousins. This girl (whose name I cannot remember for the life of me) was quite chubby, but was blessed with a superb, large set of tits which were accented by the swimsuit she wore which was too small for her. As her dad left, he told her he expected her to behave herself while looking right at me. I'd almost come in my shorts when I first saw the girl, but I hadn't thought it was THAT obvious.
Anyway, the girl and I went swimming in the lake and after awhile floated around in the deep water hanging onto a large rubber tube. We had to stay within sight of the wharf (occupied by perhaps 15-20 people), but far enough away to do what we were about to do.
Fourteen-year-old hormones were flowing freely and while the girl and I floated around, drifting in closer to the wharf and then kicking our legs to get the tube out further, I 'accidentally' let my hand brush up against her big, soft tits. This happened a few times as we talked, until finally the girl said to go ahead and feel her up.
As the tube rotated in the wind, obscuring the two of us from the people on the wharf, I slipped my hand down the front of the girl's swimsuit and got a handful of her amazing tits. Her big nipples were gloriously rock-hard. As we came back into view, I had to quickly extract my hand. We repeated this several times and my balls were set to burst. This was accentuated when I noticed the girl was rubbing her cunt through the outside of her swimsuit. We pretended to be carrying on a conversation, our eyes locked onto the others, as she jilled away and eventually came.
Luckily for me, I was swimming in my cut-offs, so I unzipped, and extracted my hard cock. The girl said to take my hand off my 'thing' because she wanted to touch one. I did, and she shyly held my cock in a gentle full-fist grip. After a bit, I couldn't take it anymore and kind of humped my cock back and forth into her hand. She caught on quick and began slowly stroking my cock.
Being a horny teenager, I would have blown my load within seconds, but we had to keep the tube at an optimum distance from the wharf and the girl had to stop jacking me when we spun into view of the wharf. This meant I got to the verge of orgasm about ten times, which built up my load and sexual tension immensely.
Finally, as we slowly spun out of sight of the wharf, the girl grasped my cock harder and wanked it faster. That did it-looking down into the clear lake water, we both enjoyed the sight of spurt after spurt of cum erupting hard from my cock. The load was immense and shot out quite far, and somehow I managed not to scream from the intense pleasure as I almost blew my nuts right off.
I got my cock back into my cutoffs and we discretely dispersed the cum by waving our hands through it underwater. Once my boner subsided, we swam back in. One of my aunts on the wharf asked if 'you teenage troublemakers' had a nice swim.
Later that evening, after a family barbecue, we were able to slip away for a few minutes and she let me dry hump my cock against the crotch of her swimsuit while squeezing her titties from the outside until I came again.